1. Field of the Invention
The present invention relates primarily to goniometer circuits and more particularly to goniometer circuits having extremely low modulation harmonic distortion.
2. Description of the Prior Art
The use of electronic goniometer circuits in VOR navigation systems has long been well known. The goniometer circuit is used with a plurality of antennas so that the direction of maximum signal output may be rotated in azimuth without physically moving the antenna array. Although existing electronic goniometers have been generally successful in attaining their intended function, it has been very difficult to eliminate modulation harmonic distortion from the output of goniometer circuits, due to the nature of the modulation required within the goniometer circuit. As is well known to those skilled in the art, modulation harmonic distortion is highly undesirable in navigation systems and can result in serious navigation errors. Accordingly a need exists for an improved goniometer circuit wherein modulation harmonic distortion is substantially eliminated.
The problem of eliminating modulation harmonic distortion is a complex one that has not been satisfactorily dealt with in the past. As an example of a previously known electronic goniometer circuit, attention is directed to U.S. Pat. No. 3,553,590, issued Jan. 5, 1971. According to this reference, and referring particularly to the embodiment of the invention shown in FIG. 4 thereof, upper and lower sideband signals are produced by an appropriate sequence of signal modulations. In the course of such modulations, however, sum and difference frequencies are produced, as is well known to those skilled in the art. These sum and difference frequencies include the desired sidebands as well as multiples of these sideband frequencies. These multiples are undesirable harmonics of the primary sideband signals which produce modulation harmonic distortion. In the above-referenced patent, the sideband signals, along with the harmonic distortion that they inherently contain, are passed through power amplifiers to an output hybrid circuit where the final modulation takes place. After power amplification, the sideband signals are detected and fed to a phase bridge which feeds back a DC error voltage to the initial modulation phase of the disclosed system to maintain a degree of phase locking. However the system disclosed in this patent fails to eliminate the undesirable modulation harmonics from the initially modulated signal and these undesirable modulation harmonics are amplified in power along with the desirable components of the signal. The phase correcting circuit is insufficient to fully eliminate the undesirable harmonics from the output signal. Accordingly, the prior art system as represented by this patent does not eliminate modulation harmonic distortion from the goniometer output signal. It is therefore believed apparent that further improvements in goniometer circuitry are desirable.